Selenium pollution is a very serious problem in California and other areas. This is because soils in the Central Valley are seleniferrous, and irrigaton leads to high concentrations of Se being leached into agricultural drainage water (as selenate). Furthermore, industries like oil refineries and electric power plants generate a lot of Se in their effluents (as selenite and selenocyanate). Our phytoremediation research has shown that constructed wetlands are very efficient at removing Se from agricultural drainage water (i.e., Corcoran, CA) and industrial effluents. Plants accumulate Se in their tissues and volatilize it as a non-toxic gas. Volatilization of Se is a safe and extremely promising technology for the remediation of polluted soils and waters. However, the intermediates that are accumulated by plants, the rate limiting steps for Se remediation, and the toxicity of intermediates are unknown factors. Data we have gained from XAS speciation studies have been essential for the identification of some intermediates and one rate limiting step in Se assimilation and volatilization. We propose to carry out six experiments which will provide information that can be applied directly to enhance the clean up of soils and waters contaminated with Se.